Transformation symmetry line symmetry line of symmetry rotational symmetry Vocabulary.
Study of Longitudinal Phase Space Distribution Measurement ...Cherenkov angle on a certain position...
Transcript of Study of Longitudinal Phase Space Distribution Measurement ...Cherenkov angle on a certain position...
A. Lueangaramwong*, F. Hinode, S. Kashiwagi,
T. Muto, I. Nagasawa, S. Nagasawa, K. Nanbu, Y. Shibasaki, K. Takahashi, K. Yanagi, H. Hama
Electron Light Science Centre, Tohoku University
Study of Longitudinal Phase Space
Distribution Measurement
via a Linear Focal Cherenkov Ring Camera
OIST Beam Physics Workshop 2013 29
November 2013
• Introduction
• Method for longitudinal phase space distribution measurement – Cherenkov radiation
– Reflective optics
• Energy resolution enhancement
• Proposed experimental setup
• Numerical results
• Conclusion
2
Outline
• A test accelerator for the coherent terahertz source (t-ACTS) at Tohoku University has been constructed – to generate intense coherent terahertz (THz) radiation
from sub-picosecond electron bunches
– an advanced independently tunable cells (ITC) thermionic RF gun consisting of two uncoupled cavities was proposed
3
Layout of t-ACTS project at Tohoku University
Introduction
• electron beam will be introduced from the RF-gun into the bunch compression system
• To obtain extremely short electron bunch production – proper longitudinal phase space distribution by the
ITC RF-gun adjusted relative RF phases and field strengths of the two cavities
4 Cross-section view of the ITC RF gun Injector part for t-ACTS
longitudinal phase space (phase dependence)
Introduction
• one of diagnostic tools to measure electron energy (electron velocity corresponds to opening angle of Cherenkov light)
• Cherenkov angle contains information of the particle energy
• aerogel (refractive index n = 1.05) = radiator
• number of the Cherenkov photons is enough to detect
5
Cherenkov Radiation
1 • e- with same Energy -> photon with same Cherenkov angle
2 • Special Mirror : “Turtle-back” mirror
3
• Focus “same-Cherenkov-Angle photon” onto one certain Position
• “different-Cherenkov-Angle photon” gives Linear Position (focal line)
4 • Streak Camera
5 • directly observe longitudinal phase space distribution
6
novel method for longitudinal phase space distribution measurement
Linear Focal Cherenkov Ring Camera
• Surface Equation: e.g. A = 35 cm;
mirror azimuthal size = 18 deg
7
• Parabolic curve : reflect the photons having the same Cherenkov angle on a certain position
• Spherical curve : designed for symmetry due to Cherenkov cone (to focus Cherenkov light on the beam axis)
A = 2x(focal length of parabolic curve) higher A => lower energy dependence
y
s
y
x
Turtle-back mirror
parabolic curve
spherical curve
Special Mirror : “Turtle-back” mirror
focal position (corresponds to electron energy)
E+dE E-dE E
e- radiator
8
• Transverse emittance – Beam size -> radiation area -> Cherenkov ring
“turtle-back” mirror cannot focus Cherenkov Ring from same energy of electron to one point
– Beam divergence -> change direction of Cherenkov rad. Direction of each electron dictates direction of Cherenkov rays which now contain information of the particle energy
• Thickness of radiator -> Cherenkov ring
radiation area & direction of Cherenkov rad
including small refraction between 2 mediums
Energy Resolution Factors
• Momentum resolution
– defined as
• Time resolution
– defined as the standard deviation of the arrival time of Cherenkov rays
9
Momentum and Time Resolution
momentum dependence at focal position
standard deviation of photon distribution in s direction
10
A Dependence of Momentum Resolution
turtle-back mirror’s parameter A
• (time resolution is nearly constant for different A)
• momentum resolution
is getting better, due to higher A (bigger mirror)
• proper beam focusing (proper twiss parameter beta) can enhance momentum resolution
momentum resolution is about 10-40 keV/c
Energy Resolution Enhancement
11
on xy plane
4x(
eff
ecti
ve f
oca
l len
gth
)
monoenergetic parallel electron beam with vertical beam size
s' new focal line
original focal line
“4f” imaging system
to transport horizontal (x) image to the new focal line
practically smaller photon distribution in s direction, or better energy resolution
on ys plane
focal point
consider Turtle back mirror in 2D
12
Energy Resolution Enhancement
4x(
effe
ctiv
e fo
cal l
engt
h)
allow turtle-back mirror’s focusing in s-direction
on ys plane
on xy plane
transport horizontal image to the new focal line
• “4f” imaging system consisted of 2 off-axis parabolic cylinder mirrors can be used to enhance energy resolution
to avoid wavelength-dependent delay and radiation damage
equivalent
13
A Dependence of Momentum Resolution
enhanced by off-axis
parabolic cylinder mirrors
turtle-back mirror’s parameter A
photon distribution at new focal line s'
• (time resolution is nearly constant for different A)
• enhancement by off-axis parabolic cylinder mirrors requires small beam divergence y’
momentum resolution is about 1-3 keV/c
14
Proposed Experimental Setup Recommended in K. Rosbach et al. Reective optical system for time-resolved electron bunch measurements at pitz. NIM A, 654, 2011.
vacuum
lower pressure
than RF cavity
-radiator chamber was proposed to put the radiator in vacuum due to effects of multiple scatterings of electrons when extracted through beam window -OAPC mirror used to transport light to outside of vacuum through quartz window
15
aerogel
e- out e- in
s=50mm
Generating Cherenkov Rays
The experimental setup with reflective optics is examined by numerical ray tracing.
transverse phase space
real space
real space
Cherenkov ring
Example of Calculation
Input electron beam
Cherenkov rays
transverse phase space
16
Ray Tracing Results
Detector screen
focal line
Its size is proportional to energy resolution by energy dependence
seems satisfied
photon distribution
17
Ray Tracing Results wavelength-dependence delay(552.225-557.775nm)
1%bandwidth of 555nm
o momentum resolution 0.019 keV/c
o time resolution 0.0014 ps
thickness of radiator
y-emittance mainly gives significant resolution • smaller beam divergence (y’)
o degrades time resolution o better momentum resolution
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 2 4 6 8 10
Thickness of Radiator(mm)
mo
men
tum
res
olu
tio
n(k
eV/c
)
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0 1 2 3 4
_
_
mo
men
tum
res
olu
tio
n(k
eV/c
)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0 1 2 3 4
tim
e re
solu
tio
n (
ps)
time resolution 0.000568 ps
small effect
(mm mrad)
(mm mrad)
• with the example setup
• for normalized emittance of 1 mm mrad with selected twiss parameters
• Sufficient momentum & time resolutions were derived
18
Discussion
Range in one shot
resolution 0.74ps is acceptable
resolution 3keV/c is acceptable
simulation
10
Turtle-back mirror
-
19
Effects of Simple Misalignment
Perfect Alignment
slightly change
off-axis on x and y : won’t become a problem off-axis on s : give error in energy measurement (position on focal line <-> energy)
We could adjust detector position
photon distribution on detector screen
• Longitudinal phase space distribution measurement via a linear focal Cherenkov ring camera has been studied
• The “4f” imaging system consisted of two off-axis parabolic cylinder mirrors can transport the Cherenkov light to outside of the vacuum system through a quartz window and enhance energy resolution of the system
• Numerical ray tracing – optimize the optical elements’ parameters and
configuration
– Sufficient energy (or momentum) and time resolutions were derived
20
Conclusion
Thank you for your kind attention
21
• number of the Cherenkov photons is enough to detect
• Example (N = 5 million) – At t-ACTS
• electron momentum extracted from ITC-RF gun 2.3 MeV/c with energy spread of about 2%
– Cherenkov radiator = aerogel • refractive index n = 1.05, thickness = 1.0 mm
• by 20 pC bunch (in micro-pulse) , p=2.3MeV/c
• 1% bandwidth of a wavelength around 555 nm – ignore frequency dependence in refractive index
22
Back up : Number of Photon
23
Back up : Energy Resolution Enhancement
24
Back up : Calculation Results not enhanced by off-axis parabolic cylinder mirrors
enhanced by off-axis
parabolic cylinder mirrors
turtle-back mirror’s parameter A
25
Back up: Calculation Results Time Resolution (ps) Momentum Resolution(keV/c)
resolution doesn’t depend on x-emittance
y-emittance mainly gives significant resolution
smaller beam divergence (y’) - degrades time resolution - better momentum resolution
1+2+5=
1+3+6=
1+4+7=